In the production of optical components, recording media, semiconductor devices, etc., as a method for forming a fine pattern in a short time, a method for forming a fine pattern on the surface of a substrate (nanoimprinting method) is known wherein a mold having on its surface a reverse pattern of such a fine pattern, is pressed to a photocurable material placed on the surface of a substrate, and the photocurable material is irradiated with light to cure the photocurable material thereby to form a fine pattern on the surface of the substrate (Patent Documents 1 and 2).
Meanwhile, in an application to an optical component (such as a lens array or a photonic crystal), the refractive index may sometimes be required to be at least 1.54. Therefore, a cured product of the photocurable material may also be required to have a high refractive index.
As a means to increase the refractive index, dispersing inorganic fine particles having a higher refractive index than an organic material in a photocurable material has been considered.
As a method of dispersing the inorganic fine particles in the photocurable material, a method of modifying the surface of the inorganic fine particles with a compound having a high affinity to the photocurable material has been known (Patent Document 3).
However, while the inorganic fine particles obtained by such a method have a good dispersibility in the photocurable material before curing, they are extruded at the time of curing the photocurable material, and then the extruded inorganic fine particles are precipitated or agglomerated, whereby the transparency of the cured product is impaired.
As a method of maintaining dispersion of the inorganic fine particles also at the time of curing the photocurable material, a method wherein inorganic fine particles modified with a compound having a high affinity are additionally modified with a compound having the same reaction site as the photocurable material is known (Patent Document 4).
The inorganic fine particles modified by such a method, have the same reaction site as the photocurable material, whereby they are fixed in the cured product in a dispersed state as is at the time of curing the photocurable material. However, a compound modifying the inorganic fine particles becomes bulky in such a method, whereby there are problems such that a refractive index as designed is not obtained and the refractive index decreases, etc.
To introduce a reaction site with a photocurable material, a method of modifying inorganic fine particles with acrylic acid has been suggested (Non-Patent Document 1).
Since acrylic acid has a low molecular weight, a compound modifying inorganic fine particles is not bulky, and therefore it seems to be a good method. However, when only acrylic acid is used, the polarity of the surface of the inorganic fine particles cannot be sufficiently covered, whereby the inorganic fine particles modified with acrylic acid have a relatively high polarity. Therefore, the following problems are caused.
(1) Since inorganic fine particles are, usually, provided as an aqueous sol, inorganic fine particles modified with acrylic acid are required to be extracted from the aqueous sol by using an organic solvent, before they are added to the photocurable material. However, the inorganic fine particles modified with acrylic acid are dispersible only in an organic solvent having a relatively high polarity (such as methanol). Therefore, filtration, solvent displacement, etc. are required before extraction, whereby steps increase.
(2) Further, the photocurable material as an object of dispersion is also limited to one having a relatively high polarity (such as a non-fluorinated (meth)acrylate).
When inorganic fine particles are to be dispersed in a photocurable material having a high hydrophobicity and containing a fluoro(meth)acrylate, a fluorinated surfactant, etc. it is required to impart an affinity to the photocurable material to the inorganic fine particles while suppressing bulkiness of a compound modifying the inorganic fine particles, and further introduce a reaction site with the photocurable material. However, as described above, by conventional methods, it is difficult to impart an affinity to the photocurable material to the inorganic fine particles while suppressing bulkiness of a compound modifying the inorganic fine particles, and further introduce a reaction site with the photocurable material.